dt-bindings: net: convert qca,ar71xx documentation to yaml

Now that we have the DT validation in place, let's convert the device tree
bindings for the Atheros AR71XX over to a YAML schemas.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>